The disclosed invention, called a video-centroid (VC) chip, relates to integrated circuits and, more specifically, comprises an analog, single integrated circuit that provides centered video images.
The centroid of an image is a very useful, well known quantity for image-tracking applications. However, present devices are digital and can provide a centered image only through the use of several integrated circuits.
Since finding the centroid is an averaging process, the solution is robust to noise as well as insensitive to minor variations in the apparent image due to changes in illumination level (but not gradient). In addition, centroid computation is consistent with a retina paradigm, i.e., it uses many simple, local computing elements in parallel to calculate a global quantity. Thus, computing the centroid of an image is a good candidate for an analog very large scale integrated (VLSI) implementation.
The earliest work on analog VLSI centroid-computing chips was reported in Deweerth, S. P., "Analog VLSI Circuits for Stimulus Localization and Centroid Computation", Int. J. Comput. Vision 8(3), 191-202 (1992). The core element for a simple centroid computation is an array of transconductance amplifiers, each of which is biased by a photodetector and generates an output current that is a function of the difference of two input voltages.
FIG. 1a is a schematic of a simple centroid-computing circuit consisting of one transconductance amplifier. As shown in FIG. 1b, the gate of one MOSFET of the input differential pair is connected to the output line (a follower), while the gate of the other is connected to a resistive divider. Applying known voltages V.sup.+ and V.sup.- on either end of the divider produces an output that varies linearly along its length and thus can be used to encode position. The left MOSFET, L, can be considered as encoding a position coordinate, while the right MOSFET, R, encodes the output position.
To perform the actual centroid computation, multiple transconductance amplifiers are configured as followers with a common position output node as shown in the schematic of FIG. 1b. The network of FIG. 1b attempts to satisfy the following equation: ##EQU1## where V(i) is the position voltage at the noninverting (+) input of the ith amplifier, I.sub.photo (i) is the corresponding ith photo-current, and U.sub.th is the thermal voltage divided by .kappa.. The slope of the tanh function is the transconductance of the amplifier. For small signals, tanh can be replaced by the transconductance, which is linear with the bias current (i.e., photocurrent) in the subthreshold region.
If Kirchoff's current law (conservation of charge) is applied at the output node, it is readily apparent that the solution to Eq. 1 is the centroid of the photocurrent distribution. The network stabilizes at a point where the output V.sub.out is equal to the solution of Eq. 1, which is the image centroid for small signals or the weighted median for large signals. Both the centroid and the median yield excellent position estimates for images with a well-defined centroid.
Thus, an analog, single integrated circuit would be desirable for computing the centroid of an image for use in image-tracking applications.